This invention relates to a closed-cell rigid polymer foam prepared in the presence of a physical blowing agent comprising a C.sub.2-6 polyfluorocarbon compound containing no chlorine or bromine atoms.
The usefulness of foamed plastic materials in a variety of applications is well-known. Rigid closed-cell polymer foams such as, for example, polyurethanes and polyisocyanurate foams are widely used as insulating structural members.
The good insulative properties of such foams are provided for by firstly the fact that they are fine closed-celled foams and secondly the closed-cell contains within a gas mixture which has a high thermal resistance or in the alternative a low thermal conductivity.
Generally, polyurethane and polyisocyanurate foams are prepared by reacting an organic polyisocyanate with an active hydrogen-containing compound in the presence of a blowing agent or agents. Generally, such blowing agents are inert organic compounds which do not decompose or react during the polymerization reaction and which as a result of the exothermic reaction if not already in the gaseous phase become converted to a gaseous phase. The gas becomes encapsulated in the liquid phase of the polymerizing reaction mixture resulting in the formation of cells, causing the reaction mixture to expand and form a foam which subsequently cures to become a rigid closed-cell foam.
Frequently used blowing agents, until recently, have been the fully halogenated chlorofluorocarbons, especially trichlorofluoromethane (Refrigerant, R-11). However, the continued use of such chlorofluorocarbon blowing agents is undesirable in view of the current opinion that their presence in earth's upper atmosphere may be a contributory factor in the recently observed reduction of the ozone concentrations.
The current commercial trend for the production of such closed-cell polymer foam is to replace the fully halogenated chlorofluorocarbons with hydrogen-containing chlorofluorocarbon compounds. These alternative blowing agents are selected as they have been identified as having significantly lower ozone depletion potentials relative to R-11. Such alternative hydrogen-containing blowing agents include dichlorotrifluoroethane (R-123), dichlorofluoroethane (R-141b), chlorodifluoromethane (R-22) and difluorochloroethane (R-142b), the use of which in the preparation of polyurethane foam has been described: see, for example, U.S. Pat. Nos. 4,076,644: 4,264,970: and 4,636,529.
However, a significant disadvantage of replacing the R-11 gas contained within the cells of the foam by such alternative compounds is a frequent loss in the initial and aged thermal insulation performance of the foam. Such loss occurs due to the generally higher gas thermal conductivities of the replacement blowing agents.
If insulative foam, especially polyurethane foam is to remain commercially attractive and be able to comply with various national standards relating to energy consumption, it is important that the foam is able to retain a good thermal insulation performance with time. This is especially critical where, because of other factors dictating the selection of blowing agents, the initial thermal conductivities of the foam may already be relatively high.
The thermal insulation properties of closed-cell foam, especially polyurethane and polyisocyanurate foam are known to become inferior with time. The loss of thermal insulation properties of a foam generally results from diffusion into the closed cells of high thermal conductivity gases, particularly nitrogen and oxygen or alternatively loss of cell gas having lower thermal conductivities. It is therefore desirable to provide for a means of limiting such loss.
One possible means to prevent loss of thermal insulation properties would be to use, for example, a gas impermeable barrier surrounding the foam. However, in most foam applications this is not a practical solution.
Alternatively, the foam could be modified to minimize or prevent loss of thermal insulating efficiency with time. With respect to this latter approach, the open literature contains relatively few teachings as to how rigid, closed-cell polymer foams might be modified to give products exhibiting an enhanced retention, or minimized loss, of thermal insulation performance with time.
U.S. Pat. No. 4,795,763 discloses carbon black-filled polyurethane foam exhibiting improved aged-thermal insulation properties. Japanese Patent Application No. 57-147510 discloses the use of carbon black to provide for lower initial thermal conductivities of the foam. The selection of graphite over carbon black for the preparation of foam from a thermoplastic resin having increased initial thermal insulation properties is disclosed by Japanese Patent Application No. 63-183941.
However, the use of fillers such as, for example, carbon black and graphite for enhancement of foam thermal insulation properties is not always possible as other physical properties of the resulting foam and processibility leading to the foam may suffer. Particularly, a high filler content can lead to highly friable and open-celled foams. Open-celled foams do not provide the desirable thermal insulation performance normally offered by closed-cell foams.
It is therefore desirable to consider the use of alternative blowing agents which provide for closed-cell foam having improved thermal insulation properties whilst additionally maintaining the overall desirable foam physical properties and processibility.
In the art, the term "thermal insulation" may be interchanged with the term "K-factor" or "thermal resistance" when discussing thermal physical properties of foams and gases.